Single pressure compensation systems for variable displacement motors are known wherein the system pressure, representative of torque load on the motor, is used to modulate the displacement of the hydraulic motor in a manner to reduce speed, and thus overload, upon an increase in sensed pressure. A motor pressure compensator allows the motor to remain at minimum displacement, and thus maximum speed, until the output load on the motor shaft increases to the level which raises system pressure to the pressure compensator setting. As the output load is further increased, the motor displacement is increased, allowing motor load torque to be supplied at a lower rpm with little or no increase in system pressure. Likewise, as the output load and system pressure are decreased, the pressure compensator causes the motor displacement to be decreased allowing the load torque to be supplied at a higher rpm. In normal operation, the pressure compensator always tries to cause the motor to operate at the compensator setting and thus at the highest speed without the pressure exceeding a preset limit. It is desirable to set the compensator at an intermediate pressure level to prolong transmission life. Thus, for a hydrostatic transmission system having a 6,000 psi maximum intermittent pressure, limited by pressure relief valves, it is usually desirable to set the motor compensator at about 3,500 psi. However, since pressures above 3,500 psi cause the motor displacement to increase, a pressure compensator system prevents getting maximum utilization of the motor even for intermittent use, that is maximum speed at maximum pressure.
In some vehicles, particularly dual track vehicles, it is common to have a dual drive hydrostatic propel system with each drive having its own independent hydrostatic transmission. If the control is of the motor pressure compensator type, each motor is subject to its own independent pressure compensator usually set at some intermediate pressure level as indicated above. In some applications, such as a snow groomer application, this intermediate pressure compensator setting causes problems during downhill operations. When the snow groomer travels downhill at high speeds and begins to slide out of control due to a lack of traction, particularly under one of the tracks, it may be necessary to operate the opposite track at full speed with high tractive effort, which requires maximum system pressure, in order to regain vehicle steering control. If the pressure compensators of the transmission motors are set at an intermediate pressure setting, such as 3500 psi, the increased tractive effort needed may be sufficient to cause the pressure compensator to operate which increases motor displacement and reduces motor speed. This reduces the capability of the high tractive effort track to reach sufficient speed to catch up with the sliding track in a manner which provides steering stability. This tractive effort and speed limitation of the driving track can be substantially eliminated by increasing the intermittent pressure compensator setting to 5000 psi, but such a high pressure setting is undesirable for continuous uphill operation since continuous motor operation at this higher pressure setting significantly reduces motor life and can cause excessive loading on the vehicle engine. The present invention is directed to providing a control system which solves the dilemma on how to meet both requirements. The snow groomer application is only one of many applications where the present invention can be used in an advantageous manner.